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       Snort - open source network intrusion detection system


       snort [-abCdDeINopqsvVxX?] [-A alert-mode ] [-c rules-file
       ] [-F bpf-file ] [-g grpname ] [-h home-net ]  [-i  inter­
       face ] [-l log-dir ] [-L bin-log-file ] [-M smb-hosts-file
       ] [-n packet-count ] [-r tcpdump-file  ]  [-S  n=v  ]  [-t
       chroot_directory ] [-u usrname ] expression


       Snort  is  an open source network intrusion detection sys­
       tem, capable of performing real-time traffic analysis  and
       packet  logging  on  IP networks.  It can perform protocol
       analysis, content searching/matching and can  be  used  to
       detect  a  variety  of  attacks and probes, such as buffer
       overflows, stealth port scans, CGI attacks, SMB probes, OS
       fingerprinting  attempts,  and  much  more.   Snort uses a
       flexible rules language to describe traffic that it should
       collect  or  pass, as well as a detection engine that uti­
       lizes a modular plugin architecture.   Snort  also  has  a
       modular   real-time   alerting  capability,  incorporating
       alerting and logging plugins  for  syslog,  a  ASCII  text
       files,  UNIX sockets, WinPopup messages to Windows clients
       using Samba's smbclient,  database  (Mysql/PostgreSQL/Ora­
       cle/ODBC) or XML.

       Snort  has  three  primary  uses.   It  can  be  used as a
       straight packet sniffer like tcpdump(1), a  packet  logger
       (useful  for network traffic debugging, etc), or as a full
       blown network intrusion detection system.

       Snort logs packets  in  tcpdump(1)  binary  format,  to  a
       database or in Snort's decoded ASCII format to a hierarchy
       of logging directories that are  named  based  on  the  IP
       address of the "foreign" host.


       -A alert-mode
              Alert  using the specified alert-mode.  Valid alert
              modes include fast, full, none, and  unsock.   Fast
              writes alerts to the default "alert" file in a sin­
              gle-line, syslog style alert message.  Full  writes
              the alert to the "alert" file with the full decoded
              header as well as the alert  message.   None  turns
              off  alerting.  Unsock is an experimental mode that
              sends the alert information out over a UNIX  socket
              to another process that attaches to that socket.

       -a     Display ARP packets when decoding packets.

       -b     Log  packets in a tcpdump(1) formatted file.    All
              packets are logged in their native binary state  to
              a  tcpdump  formatted log file named with the snort
              start  timestamp  and  "snort.log".   This   option
              results in much faster operation of the program
               since  it doesn't have to spend time in the packet
              binary->text converters.  Snort can keep up  pretty
              well with 100Mbps networks in "-b" mode.  To choose
              an alternate name for the binary log file, use  the
              "-L" switch.

       -c config-file
              Use the rules located in file config-file.

       -C     Print  the  character  data from the packet payload
              only (no hex).

       -d     Dump the application  layer  data  when  displaying
              packets in verbose or packet logging mode.

       -D     Run  Snort  in  daemon  mode.   Alerts  are sent to
              /var/log/snort/alert unless otherwise specified.

       -e     Display/log the link layer packet headers.

       -F bpf-file
              Read BPF filters from bpf-file.  This is handy  for
              people  running  Snort  as  a SHADOW replacement or
              with a love of super complex BPF filters.  See  the
              "expressions"  section  of  this  man page for more
              info on writing BPF fileters.

       -g <grpname>
              Change the GID Snort runs under to <grpname>  after
              initialization.   This  switch allows Snort to drop
              root priveleges after it's initialization phase has
              completed as a security measure.

       -h home-net
              Set  the "home network" to home-net.  The format of
              this address variable is a network  prefix  plus  a
              CIDR  block,  such  as   Once this
              variable is set, all decoded packet logging will be
              done  relative  to  the home network address space.
              This is useful because of the way that  Snort  for­
              mats  its  ASCII  log data.  With this value set to
              the local  network,  all  decoded  output  will  be
              logged  into decode directories with the address of
              the foreign computer as the directory  name,  which
              is very useful during traffic analysis.

       -i interface
              Sniff packets on interface.

       -I     Print out the receiving interface name in alerts.

       -l log-dir
              Set  the  output logging directory to log-dir.  All
              plain text alerts and  packet  logs  go  into  this
              directory.   If  this  option is not specified, the
              default logging directory is set to /var/log/snort.

       -L binary-log-file
              Set  the filename of the binary log file to binary-
              log-file.  If this switch is not used, the  default
              name  is  a timestamp for the time that the file is
              created plus "snort.log".

       -M smb-hosts-file
              Send WinPopup messages to the list of  workstations
              contained  in  the  smb-hosts-file  .   This option
              requires Samba to be resident and in  the  path  of
              the machine running Snort.  The workstation file is
              simple: each line of the file contains the SMB name
              of the box to send the message to.

       -n packet-count
              Process packet-count packets and exit.

       -N     Turn  off packet logging.  The program still gener­
              ates alerts normally.

       -o     Change the order in which the rules are applied  to
              packets.   Instead of being applied in the standard
              Alert->Pass->Log order, this  will  apply  them  in
              Pass->Alert->Log order.

       -O     Obfuscate  the  IP  addresses  when in ASCII packet
              dump mode.  This switch changes  the  IP  addresses
              that   get   printed  to  the  screen/log  file  to
              "xxx.xxx.xxx.xxx".  If the homenet  address  switch
              is  set (-h), only addresses on the homenet will be
              obfuscated while non- homenet IPs will be left vis­
              ible.   Perfect  for posting to your favorite secu­
              rity mailing list!

       -p     Turn off promiscuous mode sniffing.

       -q      Quiet operation. Don't display banner and initial­
              ization information.

       -r tcpdump-file
               Read   the  tcpdump-formatted  file  tcpdump-file.
              This will cause Snort to read and process the  file
              fed to it.  This is useful if, for instance, you've
              got a bunch of SHADOW files that you want  to  pro­
              cess  for content, or even if you've got a bunch of
              reassembled packet fragments which have been  writ­
              ten into a tcpdump formatted file.

       -s     Send  alert  messages  to  syslog.  On linux boxen,
              they will appear in /var/log/secure,  /var/log/mes­
              sages on many other platforms.

       -S n=v Set variable name "n" to value "v".  This is useful
              for setting the value of a defined variable name in
              a  Snort  rules  file  to  a command line specified
              value.  For instance,  if  you  define  a  HOME_NET
              variable name inside of a Snort rules file, you can
              set this value from it's predefined  value  at  the
              command line.

       -t chroot
              Changes Snort's root directory to chroot after ini­
              tialization.  Please note that all log/alert  file­
              names  are  relative  to  the  chroot  directory if
              chroot is used.

       -u uname
              Change the UID Snort runs under to uname after ini­

       -v     Be  verbose.   Prints  packets  out to the console.
              There is one big problem with  verbose  mode:  it's
              slow.   If you are doing IDS work with Snort, don't
              use the -v switch, you WILL drop packets.

       -V     Show the version number and exit.

       -X     Dump the raw  packet  data  starting  at  the  link
              layer.  This switch overrides the -d switch.

       -?     Show the program usage statement and exit.

              selects  which  packets  will  be  dumped.   If  no
              expression is given, all packets on the net will be
              dumped.   Otherwise, only packets for which expres­
              sion is `true' will be dumped.

              The expression consists of one or more  primitives.
              Primitives  usually  consist of an id (name or num­
              ber) preceded by one or more qualifiers.  There are
              three different kinds of qualifier:

              type   qualifiers  say  what  kind  of thing the id
                     name or number refers  to.   Possible  types
                     are  host,  net and port.  E.g., `host foo',
                     `net 128.3', `port 20'.  If there is no type
                     qualifier, host is assumed.

              dir    qualifiers  specify  a  particular  transfer
                     direction  to  and/or  from  id.    Possible
                     directions  are src, dst, src or dst and src
                     and dst.  E.g., `src foo', `dst net  128.3',
                     `src  or dst port ftp-data'.  If there is no
                     dir qualifier, src or dst is  assumed.   For
                     `null' link layers (i.e. point to point pro­
                     tocols such as slip) the  inbound  and  out­
                     bound  qualifiers  can  be used to specify a
                     desired direction.

              proto  qualifiers restrict the match to a  particu­
                     lar  protocol.   Possible protos are: ether,
                     fddi,  ip,  arp,  rarp,  decnet,  lat,  sca,
                     moprc, mopdl, tcp and udp.  E.g., `ether src
                     foo', `arp net 128.3', `tcp  port  21'.   If
                     there  is  no proto qualifier, all protocols
                     consistent with the type are assumed.  E.g.,
                     `src  foo'  means  `(ip  or arp or rarp) src
                     foo' (except the latter is  not  legal  syn­
                     tax),  `net  bar' means `(ip or arp or rarp)
                     net bar' and `port 53' means `(tcp  or  udp)
                     port 53'.

              [`fddi'  is  actually  an  alias  for  `ether'; the
              parser treats them  identically  as  meaning  ``the
              data  link  level  used  on  the  specified network
              interface.''  FDDI  headers  contain  Ethernet-like
              source and destination addresses, and often contain
              Ethernet-like packet types, so you  can  filter  on
              these FDDI fields just as with the analogous Ether­
              net  fields.   FDDI  headers  also  contain   other
              fields,  but  you  cannot name them explicitly in a
              filter expression.]

              In addition to the above, there  are  some  special
              `primitive' keywords that don't follow the pattern:
              gateway, broadcast, less,  greater  and  arithmetic
              expressions.  All of these are described below.

              More  complex  filter  expressions  are built up by
              using the words and, or and not to  combine  primi­
              tives.   E.g.,  `host  foo and not port ftp and not
              port ftp-data'.  To save typing,  identical  quali­
              fier lists can be omitted.  E.g., `tcp dst port ftp
              or ftp-data or domain' is exactly the same as  `tcp
              dst  port  ftp  or tcp dst port ftp-data or tcp dst
              port domain'.

              Allowable primitives are:

              dst host host
                     True if the  IP  destination  field  of  the
                     packet  is  host,  which  may  be  either an
                     address or a name.

              src host host
                     True if the IP source field of the packet is

              host host
                     True  if either the IP source or destination
                     of the packet is host.   Any  of  the  above
                     host  expressions  can be prepended with the
                     keywords, ip, arp, or rarp as in:
                          ip host host
                     which is equivalent to:
                          ether proto \ip and host host
                     If  host  is  a  name   with   multiple   IP
                     addresses,  each address will be checked for
                     a match.

              ether dst ehost
                     True if the ethernet destination address  is
                     ehost.   Ehost  may  be  either  a name from
                     /etc/ethers or a number (see ethers(3N)  for
                     numeric format).

              ether src ehost
                     True  if  the  ethernet  source  address  is

              ether host ehost
                     True if either the ethernet source or desti­
                     nation address is ehost.

              gateway host
                     True  if  the packet used host as a gateway.
                     I.e., the  ethernet  source  or  destination
                     address  was  host but neither the IP source
                     nor the IP destination was host.  Host  must
                     be   a  name  and  must  be  found  in  both
                     /etc/hosts and /etc/ethers.  (An  equivalent
                     expression is
                          ether host ehost and not host host
                     which  can be used with either names or num­
                     bers for host / ehost.)

              dst net net
                     True if the IP destination  address  of  the
                     packet  has a network number of net. Net may
                     be either a name  from  /etc/networks  or  a
                     network    number   (see   networks(4)   for

              src net net
                     True if the IP source address of the  packet
                     has a network number of net.

              net net
                     True  if either the IP source or destination
                     address of the packet has a  network  number
                     of net.

              net net mask mask
                     True  if the IP address matches net with the
                     specific netmask.  May be qualified with src
                     or dst.

              net net/len
                     True if the IP address matches net a netmask
                     len bits wide.  May be qualified with src or

              dst port port
                     True  if  the packet is ip/tcp or ip/udp and
                     has a destination port value of  port.   The
                     port  can  be  a  number  or  a name used in
                     /etc/services (see tcp(4P) and udp(4P)).  If
                     a  name  is  used,  both the port number and
                     protocol  are  checked.   If  a  number   or
                     ambiguous name is used, only the port number
                     is checked (e.g., dst port  513  will  print
                     both  tcp/login traffic and udp/who traffic,
                     and port domain will print  both  tcp/domain
                     and udp/domain traffic).

              src port port
                     True  if  the packet has a source port value
                     of port.

              port port
                     True if either  the  source  or  destination
                     port  of  the  packet  is  port.  Any of the
                     above port expressions can be prepended with
                     the keywords, tcp or udp, as in:
                          tcp src port port
                     which  matches only tcp packets whose source
                     port is port.

              less length
                     True if the packet has a length less than or
                     equal to length.  This is equivalent to:
                          len <= length.

              greater length
                     True if the packet has a length greater than
                     or equal to length.  This is equivalent to:
                          len >= length.

              ip proto protocol
                     True if the packet  is  an  ip  packet  (see
                     ip(4P)) of protocol type protocol.  Protocol
                     can be a number or one of  the  names  icmp,
                     igrp,  udp, nd, or tcp.  Note that the iden­
                     tifiers tcp, udp, and icmp are also keywords
                     and must be escaped via backslash (\), which
                     is \\ in the C-shell.

              ether broadcast
                     True if the packet is an ethernet  broadcast
                     packet.  The ether keyword is optional.

              ip broadcast
                     True  if  the  packet  is  an  IP  broadcast
                     packet.  It checks for both  the  all-zeroes
                     and   all-ones  broadcast  conventions,  and
                     looks up the local subnet mask.

              ether multicast
                     True if the packet is an ethernet  multicast
                     packet.   The  ether  keyword  is  optional.
                     This is shorthand for `ether[0] & 1 != 0'.

              ip multicast
                     True  if  the  packet  is  an  IP  multicast

              ether proto protocol
                     True  if  the packet is of ether type proto­
                     col.  Protocol can be a  number  or  a  name
                     like  ip,  arp, or rarp.  Note these identi­
                     fiers are also keywords and must be  escaped
                     via  backslash  (\).   [In  the case of FDDI
                     (e.g., `fddi protocol  arp'),  the  protocol
                     identification  comes from the 802.2 Logical
                     Link Control (LLC) header, which is  usually
                     layered  on top of the FDDI header.  Tcpdump
                     assumes,  when  filtering  on  the  protocol
                     identifier, that all FDDI packets include an
                     LLC header, and that the LLC  header  is  in
                     so-called SNAP format.]

              decnet src host
                     True  if  the DECNET source address is host,
                     which  may  be  an  address  of   the   form
                     ``10.123'',  or a DECNET host name.  [DECNET
                     host  name  support  is  only  available  on
                     Ultrix  systems  that  are configured to run

              decnet dst host
                     True if the DECNET  destination  address  is

              decnet host host
                     True   if   either   the  DECNET  source  or
                     destination address is host.

              ip, arp, rarp, decnet
                     Abbreviations for:
                          ether proto p
                     where p is one of the above protocols.

              lat, moprc, mopdl
                     Abbreviations for:
                          ether proto p
                     where p is one of the above protocols.  Note
                     that  Snort  does  not currently know how to
                     parse these protocols.

              tcp, udp, icmp
                     Abbreviations for:
                          ip proto p
                     where p is one of the above protocols.

              expr relop expr
                     True if the relation holds, where  relop  is
                     one  of  >, <, >=, <=, =, !=, and expr is an
                     arithmetic expression  composed  of  integer
                     constants  (expressed in standard C syntax),
                     the normal binary operators [+, -, *, /,  &,
                     |],  a  length  operator, and special packet
                     data accessors.  To access data  inside  the
                     packet, use the following syntax:
                          proto [ expr : size ]
                     Proto  is one of ether, fddi, ip, arp, rarp,
                     tcp, udp, or icmp, and indicates the  proto­
                     col layer for the index operation.  The byte
                     offset, relative to the  indicated  protocol
                     layer,  is  given by expr.  Size is optional
                     and indicates the number  of  bytes  in  the
                     field  of  interest;  it  can be either one,
                     two, or four,  and  defaults  to  one.   The
                     length  operator,  indicated  by the keyword
                     len, gives the length of the packet.

                     For example, `ether[0] & 1 != 0' catches all
                     multicast  traffic.  The expression `ip[0] &
                     0xf  !=  5'  catches  all  IP  packets  with
                     options.  The expression `ip[6:2] & 0x1fff =
                     0' catches only unfragmented  datagrams  and
                     frag  zero  of  fragmented  datagrams.  This
                     check is implicitly applied to the  tcp  and
                     udp  index operations.  For instance, tcp[0]
                     always means  the  first  byte  of  the  TCP
                     header, and never means the first byte of an
                     intervening fragment.

              Primitives may be combined using:

                     A  parenthesized  group  of  primitives  and
                     operators  (parentheses  are  special to the
                     Shell and must be escaped).

                     Negation (`!' or `not').

                     Concatenation (`&&' or `and').

                     Alternation (`||' or `or').

              Negation has highest precedence.   Alternation  and
              concatenation  have  equal precedence and associate
              left to right.  Note that explicit and tokens,  not
              juxtaposition,  are now required for concatenation.

              If an identifier is given without  a  keyword,  the
              most recent keyword is assumed.  For example,
                   not host vs and ace
              is short for
                   not host vs and host ace
              which should not be confused with
                   not ( host vs or ace )

              Expression  arguments  can  be  passed  to Snort as
              either a single argument or as multiple  arguments,
              whichever  is  more  convenient.  Generally, if the
              expression contains  Shell  metacharacters,  it  is
              easier  to  pass  it  as a single, quoted argument.
              Multiple arguments  are  concatenated  with  spaces
              before being parsed.


       Snort  uses  a  simple  but  flexible  rules  language  to
       describe network packet signatures and associate them with
       actions.   The  current  rules  document  can  be found at


       The following signals have the specified effect when  sent
       to the daemon process using the kill(1) command:

       SIGHUP Causes  the  daemon  to  close all opened files and
              restart.  Please note that this will only  work  if
              the full pathname is used to invoke snort in daemon
              mode, otherwise snort will just exit with an  error
              message being sent to syslogd(8)

              Causes  the program to dump its current packet sta­
              tistical information to the cosole or syslogd(8) if
              in daemon mode.

       Any  other  signal  causes  the daemon to close all opened
       files and exit.


       Snort has been freely  available  under  the  GPL  license
       since 1998.


       Snort  returns  a 0 on a successful exit, 1 if it exits on
       an error.


       Send   bug    reports    to    roesch@clark.net,    snort-


       Martin Roesch <roesch@clark.net>


       tcpdump(1), pcap(3)

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